The advent of next-generation 5G standard for millimeter-wave cellular communication has pushed the design of hybrid multiple-input multiple-output (MIMO) transceiver systems for interfacing with antenna array to provide directional signal transmission or reception. Signal processing techniques, such as beam forming or beam steering that can provide for the necessary spatial filtering, are implemented by a combination of digital and analog circuit blocks in the transceiver systems. Such transceiver systems may be based on time division duplex (TDD) mode of operation where transmission and reception are performed via a plurality of transmit paths and receive paths that operate at a same frequency but are activated at different time slots.
Performance in beam forming/steering may be dependent on accuracy by which the transceiver can generate phase and amplitude information to the many elements of the antenna array, and/or on accuracy by which the transceiver can extract phase and amplitude information from the many elements of the antenna array. Accordingly, calibration of a phase and amplitude adjustment block in the plurality of transmit and receive paths of the transceiver becomes a requirement, especially in mass production environments where wider tolerances in lower cost standard components may invariably introduce performance variation.
Some challenges associated with calibration of the transceiver include the high number of ports from/to which test signals must be read/injected, as well as the millimeter-wave nature of signals of interest. Description of some such challenges and implementation examples can be found, for example, in the above referenced U.S. Pat. No. 9,717,008. Furthermore, it may be desirable to reduce built-in test circuits related to the calibration, as such test circuits may not only increase a physical size of the transceiver but may also introduce undesired couplings between the different transmit and receive paths of the transceiver. Such couplings in turn may affect performance of the beam forming/steering, including in applications where (patch) elements of the antenna are fed with (horizontal and vertical) polarized RF signals.
Furthermore, requirements for higher power output from power amplifiers (PAs) used in transmit paths of transceivers that are used in, for example, base stations, may require the PAs to operate away from the power back-off condition (e.g., linear region). Accordingly, any such PAs may operate in a non-linear region (i.e., saturation) with resulting output phase and amplitude distortion that may negatively affect performance of the PA in exchange for higher output power and efficiency of the PA. Such trade-off between efficiency and linearity has been neutralized by current digital pre-distortion (DPD) schemes which may realize a linear response of a combined DPD and PA block by cascading the PA and its inverse block. As can be taken from the related references [1] and [2], which are incorporated herein by reference in their entirety, such DPD schemes may require a separate feedback path from the output of each PA, and therefore from each transmit path, to an input of a digital signal processor implementing the DPD functionality. It would be clear to a person skilled in the art that such feedback paths may cause the same challenges and problems associated with added circuits as described above with reference to the calibration of the phase and amplitude adjustment blocks of the transceiver.
Teachings according to the present disclosure aim to simplify complexity of transceivers used in beam forming/steering applications or other applications wherein the transceivers couple to a large number of antenna elements, thereby overcoming some of the challenges and problems associated with, for example, calibrating individual transmit and/or receive paths and/or instrumenting the transceivers for implementation of DPD schemes as described above.